This invention relates to nonlinear optical materials useful for wavelength conversion or parametric amplification of laser light in the fields of optical communication and optical information processing. More particularly, this invention relates to a molecular crystal useful for a nonlinear optical material. The invention also relates to methods and devices for performing wavelength conversion of laser light using the molecular crystal as a nonlinear optical material.
Nonlinear optical materials are today drawing increasing attention in the field of opto-electronics. Nonlinear optical materials are those materials which interact with light to exhibit a nonlinear optical response. Second-order nonlinear optical effects are exemplified by second harmonic generation (SHG) and the first-order electro-optical (EO) effect (Pockel's effect). The materials exhibiting these effects can be utilized to various devices such as frequency doubling of the laser light, electro-optical modulation and electro-optical switching. Continuative efforts, therefore, are being made to study nonlinear optical devices exhibiting these effects.
Nonlinear optical materials conventionally known to exhibit the SHG effects are inorganic substances such as lithium niobate (LiNbO.sub.3) and potassium titanyl phosphate (KTP). The studies heretofore made on wavelength conversion devices utilizing the SHG effect have also focused on these inorganic substances. In recent years, however, organic nonlinear optical materials having a conjugated .pi.-electron system have drawn the increasing attention because of their large optical nonlinearity and fast optical response, and many studies are being conducted in a search for promising materials.
Conventionally known organic nonlinear optical materials include urea, 2-methyl-4-nitroaniline (MNA), m-nitroaniline, N,N-dimethyl-2-acetylamino-4-nitroaniline (DAN), 3-methyl-4-nitropyridine-N-oxide (POM) and N-(4-nitrophenyl)-(L) (NPP). Details about organic nonlinear optical materials may be found in, for example,
(1) "Nonlinear Optical Properties of Organic and Polymeric Materials", ed. David J. Williams, ACS Symposium Series No. 233 (1983); PA1 (2) "Nonlinear Optical Properties of Organic Molecules and Crystals", Vols. 1 and 2, ed. D. S. Chemla and J. Zyss, Academic Press (1987); PA1 (3) "Yuki Hisenkei Kogaku Zairyo (Organic Nonlinear Optical Materials)", Masao Kato and Hachiro Nakanishi, CMC Press (1985); and PA1 (4) "Shin-Yuki Hisenkei Kogaku Zairyo (Advanced Nonlinear Optical Organic Materials)" Vols. I and II, T. Kobayashi, M. Umegaki, H. Nakanishi and N. Nakamura, CMC Press (1991). PA1 1) large optical nonlinearity; PA1 2) high transparency in the operating wavelength range, in particular, which is longer than 390 nm; PA1 3) high single crystallinity, leading to the production of single crystals of high quality; PA1 4) high mechanical strength to assure ease in crystal processing or in the fabrication of waveguides; PA1 5) thermal and chemical stability; PA1 6) controlled crystal growth in such a manner that molecules are aligned in such a direction that the nonlinear optical performance of the crystal can be effectively brought out when fabricating a bulk-type or waveguide-type wavelength conversion device.
The nonlinearity of organic substances having a conjugated .pi.-electron system is caused by the nonlinear polarization that occurs due to the interaction between laser light and the delocalized .pi.-electrons in the organic molecule of interest. In order to enhance the nonlinear polarization or the hyperpolarizability .beta. of the molecule, an electron-donating group (donor) or an electron-attractive group (acceptor) is introduced into the conjugated .pi.-electron system as a common technique for molecular design.
An issue with the organic compound synthesized by this molecular design technique is that it generally has a large dipole moment. Hence, owing to the dipole-dipole interaction, this organic compound tends to form a crystal having a centrosymmetric structure in the course of crystallization, in which the dipole moments of adjacent molecules cancel out each other. In this case, the SHG effects will no longer be observed. It is technically impossible, to date, to predict the crystal structure of a certain compound from its chemical structure. Therefore, in order to produce crystals having no centrosymmetric structure, empirical techniques are currently being employed, as exemplified by introducing an optically active group (chirality) or hydrogen bond into the conjugated .pi.-electron system or reducing the dipole moment of the molecule at the ground state so as to reduce the dipole-dipole interaction.
It should also be mentioned that compounds that experience a substantial charge transfer between a donor and an acceptor will shift the absorption maximum to longer wavelength, which results in the extension of the absorption wavelength region to the visible range. For the case of a wavelength conversion device combined with a semiconductor laser operating at 780-840 nm, the device will show the deterioration if it absorbs the light of the SHG wavelength of from 390 to 420 nm. Therefore, it is preferred for the materials of wavelength conversion devices to have an absorption at wavelengths shorter than that of SHG of the fundamental wavelength and thus it means that a cut-off wavelength (.lambda..sub.cut off) of 390 nm or shorter in the nonlinear optical materials is suitable for the application.
The inorganic nonlinear optical materials currently used in practical applications as exemplified by lithium niobate and KTP have the disadvantage that they are expensive while their performance for the wavelength conversion generally is not as good as that of organic materials. On the other hand, organic nonlinear optical materials have the advantage that they are inexpensive for material cost and can be synthesized fairly easily. However, generally speaking, the absorption wavelength range of such organic nonlinear optical materials, if they exhibit high conversion efficiency, extends to the visible wavelength region, and thus their crystals show a yellow or orange color. This means that those materials absorb the SH light of semiconductor lasers, and are unsuitable for the wavelength conversion devices.
Further, the organic nonlinear optical materials have the disadvantage of difficulty in obtaining large single crystals or fabricating waveguide-type wavelength conversion devices such as fiber waveguide-type, and slab and channel waveguide-type wavelength conversion devices. When fabricating a waveguide-type wavelength conversion device that employs an organic molecular crystal, the molecular arrangement in the crystal is important. In order to enhance the operating efficiency of the wavelength conversion device, the molecules must be aligned in such a direction that the nonlinear optical performance of the crystal can be effectively brought out. Therefore, in order to fabricate a high performance wavelength conversion device, the molecular arrangement in the single crystal must be taken into account.
While practically feasible organic nonlinear optical materials should possess various characteristics, the following are particularly important:
None of the organic nonlinear optical materials known to date have been found to be promising since they do not satisfy all of these performance requirements.